Progressive constriction of the central canal within the spinal column is a predictable consequence of aging. As the spinal canal narrows, the nerve elements that reside within it become progressively more crowded. Eventually, the canal dimensions become sufficiently small so as to significantly compress the nerve elements and produce pain, weakness, sensory changes, clumsiness and other manifestation of nervous system dysfunction.
Constriction of the canal within the lumbar spine is termed lumbar stenosis. This condition is very common in the elderly and causes a significant proportion of the low back pain, lower extremity pain, lower extremity weakness, limitation of mobility and the high disability rates that afflict this age group. The traditional treatment for this condition has been the surgical removal of the bone and ligamentous structures that constrict the spinal canal. Despite advances in surgical technique, spinal decompression surgery can be an extensive operation with risks of complication from the actual surgical procedure and the general anesthetic that is required to perform it. Since many of these elderly patients are in frail health, the risk of developing significant peri-operative medical problems remains high. In addition, the traditional treatment of surgical resection of spinal structures may relieve the neural compression but lead to spinal instability in a substantial minority of patients. That is, removal of the spinal elements that compress the nerves may cause the spinal elements themselves to move in an abnormal fashion relative to one another and produce pain. Should it develop, instability would require additional and even more extensive surgery in order to re-establish spinal stability. Because of these and other issues, elderly patients with lumbar stenosis must often choose between living the remaining years in significant pain or enduring the potential life-threatening complications of open spinal decompression surgery.
Recently, lumbar stenosis has been treated by the distraction—instead of resection—of those tissues that compress the spinal canal. In this approach, an implantable device is placed between the spinous processes of the vertebral bodies at the stenotic level in order to limit the extent of bone contact during spinal extension. Since encroachment upon the nerve elements occurs most commonly and severely in extension, this treatment strategy produces an effective increase in the size of the spinal canal by limiting the amount of spinal extension. In effect, the distraction of the spinous processes changes the local bony anatomy and decompress the nerves at the distracted level by placing the distracted spinal segment into slight flexion.
A number of devices that utilize this strategy have been disclosed. U.S. Pat. Nos. 6,451,020; 6,695,842; 5,609,634; 5,645,599; 6,451,019; 6,761,720; 6,332,882; 6,419,676; 6,514,256; 6,699,246 and other illustrate various spinous process distractors. Unfortunately, these patents disclosed device that distract one level at a time and often can not be applied at the L5/S1 level. Since disease at both L4/5 and L5/S1 is estimated to afflict more that 30% of this patient population, a need still remains for the development of devices that can simultaneously treat both levels.